The present invention relates to a method for fabricating a liquid crystal cell, and more particularly to a method for fabricating a liquid crystal cell in which the liquid crystal molecules are aligned by irradiating light.
Generally, a liquid crystal cell comprises two substrates and liquid crystal materials injected between these substrates, the liquid crystal materials comprising anisotropic molecules. It is necessary to orderly align liquid crystal molecules in the cell in order to obtain a uniform brightness and high contrast ratio in the liquid crystal cell. Therefore, some treatment on the substrate is carried out to provide orderly alignment of liquid crystal molecules. In FIG. 1, the director of liquid crystal molecule n is determined by a pretilt angle .theta. which corresponds to a polar angle of the director n and a pretilt angle direction .phi. which corresponds its azimuthal angle. The Cartesian coordinate of the director n of liquid crystal molecule is defined:
n=(cos.theta.cos.phi., cos.theta.sin.phi., sin.theta.).
The director n of liquid crystal is determined by controlling the pretilt angle .theta. and the pretilt angle direction .phi. by carrying out alignment process on the substrate. In this specification, the director n is substituted with a pretilt defining a pretilt angle and pretilt angle direction.
A conventional alignment process is the rubbing method demonstrated in FIG. 2. The rubbing process comprises following steps: coating a substrate 11 with an alignment layer 12 such as polyamide; and mechanically rubbing the alignment layer(FIG. 2a). Thereby, microgrooves with pretilt .theta..sub.p are formed on rubbed surface of the alignment layer (FIG. 2b). The liquid crystal molecules are thus uniformly aligned due to the intermolecular interaction between the polyimide molecules and the liquid crystal molecules.
In the above described rubbing process, however, defects are formed in the microgrooves which cause light scattering and random phase distortion. Also during the rubbing process, dust particles and electrostatic discharges are generated on the alignment layer, so that the substrate is damaged and yield is decreased.
To solve the aforementioned problems, the photoalignment method has been recently introduced. The conventional photo-alignment method is illustrated in FIG. 3. In this method, double exposure of linearly polarized light is necessary to determine a pretilt. FIG. 3a shows the first irradiation process in which linearly polarized ultraviolet light is vertically irradiated into an alignment layer 12. When polymers are irradiated with the light having the polarized direction .circle-w/dot. parallel with side chains of the polymers, cross-linking is generated between the side chains belonging different polymers. The reacted polymer becomes symmetric due to the side chains as well as the main chains of the polymer; the main chains are arranged in a perpendicular direction to the polarized direction .circle-w/dot. of light. Thereby, two pretilt directions .theta..sub.1, .theta..sub.2 are determined in the perpendicular direction to the polarized direction .circle-w/dot. of the light as shown FIG. 3a. However, the formed pretilt angle is small, typically less than 0.11.degree., and one pretilt direction is to be selected, so a second exposure must be carried out as shown in FIG. 3b.
The second irradiation process is shown in FIG. 3b. The linearly polarized light having a polarized direction {character pullout} perpendicular to the first polarized direction .circle-w/dot., is irradiated into the alignment layer 12 in the oblique direction .theta. relative to the normal line of alignment layer 12. Thereby, one pretilt angle direction .theta..sub.1 is selected and the pretilt angle is enlarged according to the oblique angle between the substrate 11 and incident direction of the second ultraviolet light. For example, when the oblique angle is 45.degree., the pretilt angle is set 0.26.degree..
The photo-alignment method has several advantages over rubbing. For example, it can prevent the defects of rubbing such as a light scattering and random phase distortion. Also during the alignment process, dust particles and electrostatic discharges are not generated in the alignment layer, so that the substrate is not damaged.
The conventional photo-alignment method, however, is complicated due to double-exposure. In addition, the range of controllable pretilt angle by the second ultraviolet light is very limited, for example, 0.15.degree. when the oblique angle is 30.degree., 0.26.degree. when the oblique angle is 45.degree., 0.30.degree. when the oblique angle is 60.degree..